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  STAINLESS STEEL ELECTROPOLISHING: 

         Stainless steels and irons are electro polished on a considerable scale. The process is particularly attractive in view of the high costs of mechanically finishing these alloys. Components treated range in size from kitchen sink units to small surgical needles; typical examples are windscreen wipers, wirework, automobile and cooker trim and surgical instruments.

The Process Principle :
Electropolishing is, in principle a reversal of the electroplating process. Metal is removed by applying a dc current to the anodically connected work piece submerged in a special acid electrolyte. Removal takes place without inducing stress and preferentially affects the surface micro-roughness. The surface becomes smooth and bright, with burrs, edges and corners being removed preferentially.

The results can be repeated with a high degree of precision so that components with tight tolerances can also be treated in safety.

Electropolished surfaces are distinguished by:
  • Improved corrosion resistance and durability
     
  • Reduced friction and wear
     
  • Easier cleaning, reduced build-up of products on the surface, reduced bacterial growth
     
  • Reduced radioactive contamination and improved decontamination behaviour
     
  • Prevention of catalytic wall reaction
     
  • Reduced pumping times to achieve high vacuum
     
  • Improved high frequency conductivity
     
  • Improved adhesion of plating deposits
     
  • Optimum reflectivity 
   

The Process Results :
                 Electropolished and mechanically produced metal surfaces have fundamental differences.

Mechanical operations such as machining, drilling, grinding and polishing will all adversely affect the structure and properties of the material surface. Contamination resulting from tool abrasion and contact with coolants, can lead to corrosion and wear. Dependent upon the degree of the manufacturing forces applied, the surface structure can be affected to a depth of 50 microns.

In contrast to this, Electro-polishing will remove metal from the component without mechanical, thermal or chemical transformation. Electro-polishing will eliminate damaged surface layers and allow the original properties of the material to be fully utilized.
 

The areas of Application:
This processes and the electro-polished surfaces produced by these processes have been employed in a wide range of varying applications in practically all fields of engineering and industry:

  • Chemical, biochemical, pharmaceutical industries
     
  • Food and beverage industries
     
  • Electrical and electronic engineering
     
  • Plant and equipment manufacture
     
  • Machine and tool manufacturing
     
  • Precision engineering and associated equipment
     
  • Textile and paper industry
     
  • Automotive and vehicle manufacturing
     
  • Aviation and aerospace engineering
     
  • Nuclear research and nuclear industry
     
  • Vacuum and cryogenic engineering
     
  • Ornaments and fancy metal goods
     
  • Architecture and interior decoration
     
  • Household and kitchen equipment like wet grinder parts, mixe parts etc,.

STAINLESS STEEL ELECTROPOLISHING SOLUTION

Solution Composition

              The electro polishing solution is supplied in a prepared form ready for use.

Equipment:

           For this solution a tank lined with chemical lead is employed.. The provision of mild air agitation is necessary in order to provide solution movement during processing and to prevent layering.. For solution heating silica cased immersion heaters may be used, alternatively a hot water jacket may be provided.

                         For cathodes, lead or copper strips 150 to 200 mm (6 to 8 in) wide are used. The cathodes should be of sufficient area to give a cathode current density of 5 to 8 amp / dm(50 to 75 amp /ft2). The cathodes will require periodic brushing to remove the scale formed upon the surface.

 JIGS :

       The use of titanium jigs is advised. Alternatively copper or brass jigs with bronze contacts may be used but these will only have a limited life. The jigs should be of adequate size in order to carry the high currents employed.

 Operating Conditions :

 Anodic Current Density.: 16 to 45amp / dm2  (150 to 400 amp / ft2) dependent on alloy    

                                         used and shape of article.

 Voltage.                        : 10 to 20volts dependent upon the size of the load, the current

                                        density applied, the anode / cathode distance and ratio of the

                                        surface areas of the anode and cathode.  

 Temperature.                :  60 to 90` C (140 to 195`F).

 Time.                           :   4 to 10 minutes. To ensure maximum solution life the life the

                                         Processing times should be kept as short as possible.

   

 Process Sequence :

The following sequence is advised:

1.      Cleaning. Solvent degrease in a solvent degreaser or, alternatively, hot clean in anodic cleaner, depending upon the surface condition. Where solvent degreasing is employed it is advisable to follow with hot alkaline cleaning..

2.      Swill

3.      Hot swill and dry.

4.      Electropolish for 4 to 10 minutes.

5.      Swill.

6.      Swill.

7.      Passivate in a 5 per cent volume sulphuric acid – 1 per cent sodium dichromate solution at room temperature, or in a 20 per cent volume nitric acid at 50` C (120`F) for a few minutes.

8.      Swill.

9.      Swill.

   10.   Hot water and dry.

 Solution Maintenance :

          The solution is controlled by adjustment of the density and the iron content.

Density

          As supplied the Stainless Steel Electropolishing Solution will having a density of approximately sp. gr. 1.75 at 15 0 C (62 0 Be). In use the density of the solution should be maintained within the following limits:               

Solution temperature

       60 0 C

          90 0 C

Density – Minimum value

Density – Maximum value

Sp. gr. 1.70 60 0Be

Sp. gr. 1.81 65 0Be

Sp. gr. 1.64 57 0 Be

Sp. gr. 1.75 62 0 Be

        In use the density of solution will normally rise. If, however, the density of the solution tends to fall, this will be due to excess carry –over of water on the work or absorption of moisture from the atmosphere. Articles should be dried before being immersed in the electropolishing solution and the tank should be covered when not in use.Excess water may be driven off the solution by heating the solution to 95to105 0 C (200 to 220 0F) for a period.

 Iron Content:

                       The degree of brightness will tend to fall when the iron content reaches 60 g /l (10 oz / gal) and the iron content should normally be kept below 50 g / l (8 oz / gal) by drag-out or the removal of a small proportion of the solution. Periodical elimination of 10 - 20 % of the total solution and replenishment with fresh solution.

       As a guide, the use of the solution to the extent of four-ampere hours per litre will increase the iron content by about 1 g/l. (114 ampere hours per gallon use will raise iron content by 1 oz/gal.)

          Most of the nickel and some of the iron dissolved from the processed components will precipitate as sludge. It will, therefore, be necessary for the tank to be emptied occasionally and the sludge removed. 

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